Intermediate filaments (IFs) are a diverse family of cytoskeletal proteins, classified into six Types- on the basis of sequence homologies. Types I and 11 are keratins, with the "hard" a-keratins coming from. epidermal specializations such as hair and hom and the "soft" a-keratins expressed in other tissues. Previous STEM measurements of mass/length on lFs showed a quaritized polymorphism but were done mostly on IFs assembled in vitro from subunits isolated under denaturing conditions so any physiological implications were unclear. A source of native "hard" a-keratin IF isolated from rat whisker follicles has motivated a reinitiation of these studies. In additon, x-ray diffraction studies and sequence-based model-building suggest that "hard" a-keratin filaments differ fi-om other IFs. There appear to be -30 q 3 chains in section.